Although a large volume of literature focuses on outcomes of extremely low birth weight infants, data are not available which clearly define the prevalence of handicapping conditions in this population during infancy or childhood. While it is apparent that mortality rates have declined dramatically over the past twenty years in this group of infants, it is not clear that morbidity has been similarly affected. Published studies offer conflicting results and conclusions. It is clear that the extremely low birth weight population has captured the national attention of health care providers, third party carriers, governmental agencies and the general population. The limits of viability as defined by gestational age and birth weight have continued to decrease, thereby prompting debate regarding the cost versus benefit of newborn intensive care for this population. Because of the considerable resources utilized in the care of this group of infants, it is imperative that accurate information regarding outcome is available so that discussions are based upon current and statistically valid data. The follow-up study is a long-term evaluation of all surviving infants weighing 401-1,000 grams at birth who are admitted to the NICU of the NICHD Neonatal Research Network centers after January 1, 1993, and who are part of the NICHD Generic Data Base. An assessment which includes all aspects of growth and development is undertaken for infants at 18 months corrected age based on best obstetric estimate. The assessment battery includes demographic and medical history, physical and neurological examination, Bayley Scales of Infant Development-II (neurodevelopmental and behavioral assessment), Functional Status II(R) (parental assessment of the child's functioning and health status), Family Resource Scale (social and economic stress measurement), and Adapted Version of Impact on Family G. Linking information from the NICHD Neonatal Research Network Generic Data Base to a follow-up assessment at 18 months provides a unique opportunity to address health policy debates as well as evaluate the impact of care practices.